Preheat your oven to 180℃ (or 160℃ for fan ovens) and line an 8x8 tin with baking paper.
Melt the butter, sugar and golden syrup together over low heat on the stove. Once the sugar has dissolved, remove it from the heat.
Mix in the oats, making sure there are no dry bits left, then stir in the white chocolate chips.
Press the mixture firmly into your tin and bake for around 20 minutes. The flapjack should be golden at the edges and wobbly in the middle. It'll firm up as it cools so don't worry.
Let it cool in the tin for around 5 minutes then use a knife to gently score lines where you plan to cut it (this will help to prevent crumbling later). Leave to cool completely, around 3-4 hours.
Remove the flapjack from the tin and drizzle the melted white chocolate on top. Leave the chocolate to harden then cut the flapjack into slices and enjoy!